This Phase II, randomized, multicenter, double-masked study will assess the ability of recombinant human interleukin eleven (NEUMEGA rhIL-11 Growth Factor) to decrease the number of platelet transfusions required by patients with breast cancer during two cycles of high-dose cyclophosphamide/doxorubicin chemotherapy, to assess the safety of rhIL-11 in these patients, and whether anti-IL-11 antibodies are produced in them.